Embodiments of the invention relate to coating systems and methods, and methods of coating the inside of pipes and other products having interior surfaces.
Electrophoretic deposition (or EPD) is a method of applying a material, such as paint, to an electrically conductive surface. For example, EPD has been widely used to coat automobile bodies and parts, tractors and heavy equipment, electrical switch gear, appliances, metal furniture, beverage containers, fasteners, and many other industrial products. Some forms of electrophoretic deposition include electrocoating, e-coating, cathodic electrodeposition, anodic electrodeposition, aqueous electrophoretic deposition, and electrophoretic coating, or electrophoretic painting.
The EPD process involves preparing the product for coating, coating the product with the main coating, and the curing the coating on the product. During the preparation stage, the product is typically cleaned and coated with a pre-coat, such as an inorganic phosphate coating, silane coating, zirconium, or any other conversion coating. When applying the main coat, the product is submerged in a reservoir filled with a solution of polymers that often includes of a mixture of the coating and water. The coating is applied by directing an electrical current through the reservoir using electrodes. The product being coated is considered one of the electrodes, and a set of “counter-electrodes” is used to complete the circuit. Typical voltages can be anywhere from 25-400 volts of direct current. Depending at least in part on the material of the product being coated, higher and lower voltages are possible.
When the voltage is applied to the system, the molecules in the coating attach to the surface of the product, which acts as one of the electrodes. More specifically, the polymer molecules carrying a certain charge will attach to the product, which carries the opposite charge as the polymers. For example, if an anodic EPD process is used, the polymers will carry a negative charge, and will be deposited on a positively charged product. In this case, the counter-electrodes act as cathodes and the product acts as the anode. On the other hand, if a cathodic EPD process is used, the polymers will carry a positive charge, and will be deposited on a negatively charged product. In this case, the counter-electrodes act as anodes, and the product acts as the cathode.
After the coating is applied to the product, excess solution is then rinsed off of the product. Finally, the coating is fixed, or cured, to the product.
EPD processes have a number of advantages that make the process appealing. For example, the applied coatings generally have a very uniform thickness. Objects with complex shapes can be easily coated. The process is fairly high speed and can apply to a wide range of materials, such as metals, ceramics, and polymers. One limitation of EPD is that it is difficult to use to use EPD to coat the inside of products having interior surfaces, such as pipes, and other products having internal cavities where the electric current cannot travel easily. Accordingly, many product manufacturers coat the inside of products with materials that are less than optimal primarily because EPD and other product coating processes are not available. By way of example, many large pipe manufacturers coat the inside surfaces of the pipes with asphalt using an alternative method, rather than EPD.